The present invention relates to the technical field of manufacturing optical elements and testing of optical elements using a measuring apparatus like, for example, an interferometer. In particular, the present invention relates to a method of measuring a shape of an optical surface of a test object, an interferometric measuring device configured for measuring a shape of an optical surface of a test object, a method of qualifying a wave shaping element having a wave shaping surface and an optical element manufactured using the above method or the above interferometric measuring device.
The optical element can for example be an optical lens or an optical mirror used in an optical system, such as a telescope used in astronomy or a projection optical system used for imaging structures, such as structures disposed on a mask or reticle, onto a radiation sensitive substrate, such as a resist on a wafer, in a microlithographic method. The performance of such an optical system largely depends on the accuracy with which the optical surface can be processed or manufactured to have a target shape determined by a designer of the optical system. In such manufacture it is necessary to compare the shape of the processed optical surface with its target shape, and to determine differences between the processed surface and the target surface. The optical surface may then be further processed at those portions at which differences between the machined and target surfaces exceed for example predefined thresholds.
In a conventional method, an optical test surface, which can be of aspherical shape, is disposed in a beam path of incoming measuring light of an interferometer. The interferometer comprises a wave shaping element, also called compensation system, that shapes the beam of the measuring light such that the measuring light is substantially orthogonally incident on the optical surface at each location thereof. Thus, the wavefront of the measuring light has substantially the same shape as the surface shape of the optical surface, on which the measuring light is orthogonally incident. Compensation systems are also referred to as null-lenses, null-lens systems, K-systems and null-correctors. Background information relating to such compensating systems is available for example from chapter 12 of the text book of Daniel Malacara “Optical Shop Testing”, 2nd edition, John Wiley & Sons, Inc. 1992.
For testing complex aspheres often computer generated holograms (CGHs) are used as compensation systems. For obtaining a highly precise measurement of the shape of the optical test surface all manufacturing errors of the elements in the cavity of the measuring interferometer have to be known precisely. Alternatively, a calibration asphere can be used for calibrating such errors. Often, however, such a calibration asphere is not available. Sometimes a CGH operated in transmission followed by a mirror is used as a calibration object for calibrating the interferometer. The accuracy of the calibration CGH, however, is not better than the accuracy of the compensation system. It is further necessary, to align the calibration CGH to the mirror, which is another source of errors.
Mid-spatial-frequency deviations of the wave front generated by a compensation system in form of a CGH can be caused e.g. by the electron beam writing process. Single writing fields are shifted relative to each other, such that the CGH-structure deviates from section to section laterally from its target position. Further reasons can be local manufacturing errors of the CGH, e.g. protruding structural parts, modified shoulder angles, foreign particles or other defects.